void reverse(char line[]){
   int i;
   int length;
   char tmp;
     ..
     ..
     ..
   return;
}

#include<stdio.h>
#include<string.h>

void reverse(char line[])
{
    int  i;
    int  length;
    char temp;
    if (line == NULL)
        return;
    length = strlen(line);
    for (i = 0 ; i < length / 2 + length % 2 ; ++i)
    {
        if (line[i] == line[length - i - 1])
            continue;
        temp                 = line[i];
        line[i]              = line[length - i - 1];
        line[length - i - 1] = temp;
    }
    return;
}




 int main()
{

    FILE *src_fh, *dst_fh;
    char src_fn[256+1], dst_fn[256+1];


    printf("Enter Source File Name:n");
    fgets(src_fn, sizeof(src_fn), stdin); reverse(src_fn);


    if( (src_fh = fopen(src_fn, "r")) == NULL )
    {
        printf("ERROR: Source File %s Failed To Open...n",src_fn);
        return(-1);
    }


    printf("Enter Destination File Name:n");
    fgets(dst_fn, sizeof(dst_fn), stdin); reverse(dst_fn);


    if( (dst_fh = fopen(dst_fn, "w+")) == NULL )
    {
        fclose(src_fh);
        printf("ERROR: Destination File %s Failed To Open...n",dst_fn);
        return(-2);
    }

    int ch;
    while( (ch = fgetc(src_fh)) != EOF )
    {
        fputc(ch, dst_fh);
    }


    fclose(src_fh);
    fclose(dst_fh);
    return 0;
}

void reverse(char line[])
{
    int  i;
    int  length;
    char temp;
    if (line == NULL)
        return;
    length = strlen(line);
    for (i = 0 ; i < length / 2 + length % 2 ; ++i)
    {
        if (line[i] == line[length - i - 1])
            continue;
        temp                 = line[i];
        line[i]              = line[length - i - 1];
        line[length - i - 1] = temp;
    }
    return;
}

void reverse(char line[])
{
    int i;
    int length;
    int half;

    if (line == NULL)
        return;
    length = strlen(line);
    half   = length / 2 + length % 2;
    for (i = 0 ; i < half ; ++i)
    {
        if (line[i] == line[length - i - 1])
            continue;
        line[length]         = line[i];
        line[i]              = line[length - i - 1];
        line[length - i - 1] = line[length];
    }
    line[length] = '';
    return;
}

the following code
1) incorporates proper error checking
2) outputs each input line, reversed, to the output file.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* chomp(char* p)
{
    int len;
    if(!p) return(p);
    if( (len=strlen(p))<=0 ) return(p);
    if( p[len-1] == 'n' ) { p[--len] = ''; }
    if( p[len-1] == 'r' ) { p[--len] = ''; }
    return(p);
} // end function: chomp

int main()
{
    /* Create Usable Variables */
    FILE *src_fh = NULL;
    FILE *dst_fh = NULL;
    char src_fn[256+1] = {''};
    char dst_fn[256+1] = {''};
    char line[2048]    = {''};

    /* Retrieve Source File Name From User */
    printf("Enter Source File Name:n");
    if( NULL == (fgets(src_fn, sizeof(src_fn), stdin) ) )
    { // fgets failed
        perror("fgets for input file name failed" );
        exit(EXIT_FAILURE);
    }

    // implied else, fgets successful

    chomp(src_fn); // remove trailing newline characters

    /* Attempt Opening Source File For Reading */
    if( (src_fh = fopen(src_fn, "r")) == NULL )
    {
        perror( "fopen failed" );
        printf("ERROR: Source File %s Failed To Open...n",src_fn);
        return(-1);
    }

    // implied else, fopen source file successful

    /* Retrieve Destination File Name From User */
    printf("Enter Destination File Name:n");
    if( NULL == (fgets(dst_fn, sizeof(dst_fn), stdin) ) )
    { // then fgets failed
        perror( "fgets for output file name failed" );
        fclose(src_fh); // cleanup
        exit( EXIT_FAILURE );
    }

    // implied else, fgets for output file name successful

    chomp(dst_fn); // remove trailing newline characters

    /* Attempt Opening Destination File For Writing */
    if( NULL == (dst_fh = fopen(dst_fn, "w")) )
    {
        perror( "fopen for output file failed" );
        fclose(src_fh); // cleanup
        printf("ERROR: Destination File %s Failed To Open...n",dst_fn);
        return(-2);
    }

    // implied else, fopen for output file successful

    int index;
    /* Copy Source File Contents (reversed, line by line) to destination file */
    while( NULL != (fgets(line, sizeof(line), src_fh) ) )
    {
        chomp(line); // remove trailing newline characters
        index = strlen(line) - 1; // -1 because arrays start with offset 0
                                  // and strlen returns offset to ''

        // output reversed line to file
        while( index >= 0 )
        {
            fputc( line[index], dst_fh );
            index--;
        } // end while

        fputc( 'n', dst_fh );
    } // end while

    /* Close Files On Success */
    fclose(src_fh);
    fclose(dst_fh);
    return 0;
} // end function: main